A graphical version of this
slide is also available. In the text only version presented here, * denotes
multiplication, / denotes division, ^ denotes exponentiation, exp denotes the
exponential function. The flow variables: temperature in degrees Centigrade
is given by T, density in kilograms per cubic meter by r, pressure in kilo Pascals
by p, and altitude in meters by h.

The Martian atmosphere is an extremely thin sheet of gas,
principally carbon dioxide, that extends from the surface of Mars to
the edge of space. The Martian atmosphere is less dense than the
Earth's atmosphere, but there are many
similarities. Gravity holds the atmosphere to the Martian surface.
And within the atmosphere, very complex chemical, thermodynamic,
and fluid dynamics effects occur. The
atmosphere is not uniform; fluid properties are constantly changing
with time and place producing weather on Mars just like on Earth.

These variations extend upward from the surface of Mars. The sun
heats the surface, and some of this heat goes into heating the gas
near the surface. The heated gas is then diffused
or convected up through the atmosphere. Thus, the gas temperature
will be highest near the surface and decreases as altitude increases.
The speed of sound
depends on the temperature and also decreases with increasing altitude.
As with the Earth, the pressure in the
atmosphere decreases with altitude. The density
of the atmosphere depends on both the temperature and the pressure
through the equation
of state
and also decreases with increasing altitude.

Aerodynamic forces directly depend on
the gas density. To help aircraft designers, it is useful to define a
mathematical model of the atmosphere to capture the effects of
altitude. The model shown here was developed from measurements of the
Martian atmosphere made by the Mars Global Surveyor in April 1996.
The information on the Martian atmosphere was gathered by Jonathon
Donadee of Canfield (Ohio) Middle School during a cyber-mentoring
program in 1999. The data was curve fit to produce equations by Dave
Hiltner of St. John's Jesuit High School as part of a shadowing
program in May 1999. The curve fits are given for Metric units.
These curve fits are also available in English
units. The model has two parts.

The first part covers the lower atmosphere
which extends from the surface to 7000 meters.
In this layer temperature decreases with altitude:

Temperature: T = -31 - .000998 * h

Pressure: p = .699 * exp (-.00009 * h)

The second part covers the upper atmosphere,
which extends upwards from 7000 meters.
In this layer the temperature also decreases:

Temperature: T = -23.4 - .00222 * h

Pressure: p = .699 * exp (-.00009 * h)

In each part, the pressure and temperature are curve fit; and the
density is derived from the equation of state. r = p / (.1921 * [T + 273.1])

An interactive
Java applet
for this slide is also available. With the
applet, you can change altitude and see the effects on pressure and
temperature.
You can also compare the Martian atmosphere to the
atmosphere on Earth.